1. Field of the Art
The present invention relates to a developing device using a magnet brush and an image forming apparatus including the same.
2. Description of the Background Art
A developing system using a magnet brush is extensively applied to a photographic image forming apparatus of the type using a two-ingredient type developer, i.e., a toner and carrier mixture. In the magnet brush type developing system, a developer carrier conveys a developer deposited thereon in the form of a magnet brush and causes it to contact an image carrier, which carries a latent image thereon. An electric field is formed between the image carrier and a sleeve to which an electric bias is applied. The electric field causes the toner of the developer to selectively deposit on the latent image for thereby developing the latent image.
The developer carrier has a magnet roller accommodated in the sleeve, which is usually cylindrical. The magnet roller causes the developer deposited on the sleeve to rise in the form a magnet brush. The toner, which is charged to preselected polarity, deposits on the carrier present in the magnet brush. The magnet roller has a plurality of magnetic poles each being formed by a particular rod-like or similar magnet. Among the poles, a main pole is positioned on the surface of the sleeve in a developing region for causing the developer to rise. At least one of the sleeve and magnet roller moves relative to the other so as to cause the developer forming the magnet brush on the sleeve to move.
The developer brought to the developing region rises in the form of chains along magnetic lines of force issuing from the main pole of the magnet roller. The chains contact the surface of the image carrier while yielding. The chains feed the toner to the latent image while rubbing themselves against the latent image on the basis of a difference in linear velocity between the developer carrier and the image carrier. The developing region refers to a range over which the magnet brush on the developer carrier contacts the image carrier.
A problem with the conventional developing system of the type described is that a brush mark appears in an image if the amount of the developer deposited on the sleeve is short. Conversely, the developer adheres to the sleeve or varies a load acting on the image carrier if excessive in amount.
The distance between the image carrier and the sleeve, i.e., a development gap contributes a great deal to image quality. The gap enhances the edge effect and thereby lowers image quality if excessive or intensifies an electric field for development if short. The intensified electric field increases the gamma of an image and thereby degrades tonality while causing the developer to adhere to the sleeve. While the amount of the developer required to deposit on the sleeve depends on the above gap, an adequate amount has not been defined specifically yet.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-222673 and 2000-305360.
It is an object of the present invention to provide a developing device that enhances image quality and reduces the variation of a load on an image carrier by defining an adequate amount of developer for a gap implementing high image quality, and an image forming apparatus including the same.
A developing device of the present invention includes an image carrier for forming a latent image thereon, and a developer carrier for depositing a developer thereon in the form of a magnet brush. The magnet brush is caused to contact the latent image to thereby develop it. The developer carrier includes a main magnetic pole for development and an auxiliary magnetic pole adjoining the main pole and helping the main pole exert a magnetic force. The developer carrier and said image carrier are spaced by a gap of 0.3 mm or above, but 0.6 mm or below.
An image forming apparatus including the above-described developing device is also disclosed.